Apparently when you versionize the IDL file, you also need to versionize the main inheritance from IDispatchImp. Parameters 4 & 5 are the Major and Minor versions of your IDL file.

Snippet from MSDN:
For dual interfaces, the ATL Object Wizard generates code that derives your COM class from IDispatchImpl. IDispatchImpls fourth and fifth arguments are default arguments that map to the major and minor versions of the type library. These default to version 1.0. ATL uses this version to call LoadRegTypeLib(), which fails with TYPE_E_LIBNOTREGISTERED if the version in the type library doesnt match.